DD slumping in booster seat

DD is 3.5 and approx 20kg and very tall. She is in a Britax Evolva 1-2-3. Because of the weight limits we are using the seat belt rather than the harness. She is normally fine but has a habit, especially when tired, of slumping in the car seat and as the seat belt doesn't have a crotch strap to keep her upright her bum can slide quite far forward in the seat.

Her seat is in the middle position of the middle row in our Kia Sedona. THere is nothing between her and the windscreen of the car. I am terrified that in the event of a collision she will submarine under the seat belt and potentially through the windscreen.

If i catch her doing it she gets read the riot act and she knows it is very naughty to do it but she is also 3.5 years old and impulse control is not a strong point. I have a 18month old and a 4 week old as well so i worry i will be distracted and wont notice she is slumping so am looking for a foolproof physical solution.

There is no way she would tolerate a new rear facing seat (i saw that the extended rear facing seat have higher weight limits for the harness than the forward facing ones). We could afford a new booster seat if we have to. i have had a look on halfords for a booster seat the has a crotch bar or something to physically stop her sliding but so far have not found anything.

Has anyone else encountered this problem and if so, how did you deal with it.

I've just been comparing pictures of your seat and the Britax KidFix we've got. Just looking at the pictures the Evolva's seat has quite a noticeable dip at the front and the lap belt guides don't look as well defined. Might be with a look at it in real life in your car to see if it holds her more snug.

Have you got the Evolva in the reclined or the upright position? I know it's not much of a recline, and in our car we had to have it in the recline position because it fitted better that way, but flipping the bit under the seat and fitting it in the upright position might help (if you haven't tried that already).It must be tricky if you have a heavy child. My DD1 has only just reached 18kgs at almost 5. And DD2 has yet to reach 9kgs at 18 months, so she'll be in a 5-point harness forever!I think the Evolva is a good seat in theory, but with it trying to cover such a wide age range there are always going to be compromises. For example, the lack of recline doesn't suit a younger child, and for an older child I don't think it works as well as a dedicated Group 2-3 high backed booster. I could never get the seatbelt to run freely through the shoulder guide, if DS leaned forward the seatbelt did not retract when he sat back and therefore was left slack across his body. I switched him to a Britax Adventure high backed booster instead and was much happier with that seat.

She might surprise you and like a rear facing seat, I always thought it was exciting to travel backwards as a kid. Other than that a lot of the ERF seats can be used FF to a higher weight limit, usually 25kg.

My 5yo is very slouchy if on a booster cushion or without a car seat, but he sits up very straight in a smaller booster like the Graco junior maxi or the Britax Adventure. Why don't you see if you can borrow one for a few days (you could lend the person your evolva) and if it works, put the evolva aside for another child and get her a more compact one.